Method of fabricating a turbine blade having a leading edge formed of weld metal

ABSTRACT

THE PRODUCTION OF TURBINE BLADES IN WHICH A BLADE BLANK IS PROVIDED IN THE AREA OF THE INLET EDGE WITH A STEEL WELD PORTION, WHICH, AFTER SMOOTHING OF ANY UNEVEN SPOTS, IS SHAPED TOGETHER WITH THE BLADE BODY TO THE FINAL DIMENSIONS IN A DROP FORGE AND FINALLY SUBJECTED TO HEAT TREATMENT.

23, 1971- J. HIRTENLECHNER 3,

METHOD OF FABRICATING' A TURBINE BLADE HAVING 'A LEADING EDGE FORMED 0FWELD METAL Filed May 27, 1968 Fig. 2

Fig. 3

In ventor I H kTEA/LEC a M54 United States Patent US. Cl. 29-156.8 3Claims ABSTRACT OF THE DISCLOSURE The production of turbine blades inwhich a blade blank is provided in the area of the inlet edge with asteel weld portion, which, after smoothing of any uneven spots, isshaped together with the blade body to the final dimensions in a dropforge and finally subjected to heat treatment.

This invention relates to the production of turbine blades.

The development in turbine construction during the last few years hasincreasingly been directed toward larger constructional units. Therequired volume of the gaseous driving agents, particularly in the finalstages, therefore becomes increasingly greater and, by the same measure,larger blade surfaces are required. The increase in dimensions of thefinal blades does not merely refer to length but, to an even greaterextent, to the width of the blade so that its is necessary in manyinstances to develop new blade shapes. The likewise greatly enlargedboundary diameter, very high revolutions per minute, and consequentialperipheral speeds of Mach 2 in the low-pressure stages, impose aconsiderable strain on the blades through strong centrifugal forces andoscillations. Especially in the case of steam turbines, considerableerosion and corrosion occurs mainly in the area of the inlet edges,because of condensation developing in the case of a lowering of thepressure.

The selection of raw materials, which should have sufficient strength,corrosion resistance and surface hardness at high operatingtemperatures, on the one hand, as well as the construction from thepoint of view of flow engineering and the correct shape of bladespursuant to the degree of strength, on the other hand, make thecommercial fabrication of blades a problem diificult to solve.

Therefore, the invention relates to a process for the manufacture ofturbine blades which will meet all these requirements which are theresult of modern technological development.

It is customary to make the blades, for example, from a blank madethrough rolling or forging, by means of cutting or noncutting treatment,or through casting the blades in their final dimensions, or slightlybeyond those dimensions according to wax melting, form masking or otherprocesses.

At the same time, raw materials must be used which have the followingcharacteristics:

(1) Sufficient mechanical strength against strain on the blades withregard to tension because of very considerable centrifugal forces, andagainst bending due to the pressure of the driving agent which often hasa vibrating effect (endurance bending strength).

(2) Creep strength at operating temperatures of 650 C. and beyond.

(3) Corrosion resistance against attack by the driving agents,particularly stress corrosion and vibration stress from corrosiveagents.

3,564,689 Patented Feb. 23, 1971 (4) The resistance of the surfaceagainst erosion and cavitation by the driving agent.

(5) Workability by cutting processes during the shaping of the bladebase.

Heretofore, one tried to meet all these requirements through the use ofsteels with a content of 13% chrome and 0.25% carbon, which steels intheir improved state have a basic martensitic structure with areas offerrite. For modern, high-speed turbines, however, neither the strengthvalues nor the behavior in the case of alternating reversal, nor thetoughness of these steels sufficed. Therefore, an attempt was made toachieve better characteristics by increasing the carbon content. At thesame time, it was important to make the inlet edges, which are under thegreatest stress, more resistant to erosion through local surfacehardening by means of flame or induction heating.

The 13% chrome steels, which, in view of the partial surface hardeninghad been more highly carbonated at the edges of the blades, are lesscorrosion resistant when they are annealed in the area of 500-650 C.and, moreover, they show essentially no improved strength values incomparison with the low carbonated types if one wants to preventunocntrollable annealing processes during the surface hardening, and ifone increases the annealing temperature beyond 650-700 C.

Also, experiments with other alloys, especially with higher chromecontents, and Cr-Ni-Mo-V-Nb steels alloyed in a complex manner, failedbecause the toughness value led to dangerous embrittlement because ofcarbide and nitride segregation in the case of certain annealingtemperatures. Also, stress and crack corrosion has produced very unevenvalues after the annealing treatments in the critical temperatureranges, just as did the fatigue limit in the case of simultaneouscorrosive attack.

If, therefore, one wants to obtain improvement in the essentialcharacteristics by means of metallurgical methods, especially throughuse of steels alloyed in a complex manner, application of thoseprocesses for the strengthening of the inlet edges which can lead, atthe zones of transition, the annealing phenomena which decrease quality,will be out of the question.

In order to overcome the described difiiculties, the invention proposesa new production process for turbine blades, which process ischaracterized in that a blade blank, produced in the customary manner,is provided at the area of the inlet edge with a steel plate portionthat is welded in pre-prepared recess, for example, the weld materialconsisting of an alloy made of hard metal of high-speed steel, orsomething similar. The portion is built up by welding and any unevenspots in the welded portion are smoothed. The weld portion is shaped tothe final dimension in a drop forge, together with the body of theblade, and the turbine blade is finally subjected to heat treatment.

This process according to the invention results in considerableadvantages as compared to all processes proposed heretofore. Both inconjunction with the shaping, which takes place subsequent to the steelplate portion being welded, as well as the subsequent heat treatment,the formation of abrupt transitions from one shape of the structure toanother will be prevented, and, as a result thereof, the development ofcritical deteriorations of the local characteristics of strength aredecreased. Through this method it is possible to adapt the protectiveedges made of hard alloys to truly every desired blade profile withoutany disturbing welding joint. The blade profile runs completely andsmoothly around the curvature of the inlet edge. Experiments by way ofcomparison with the turbine blade produced according to the tradi tionalmethods have shown that the ones produced according to the presentprocess have a considerably longer holding time and that, moreover,during the end control, a considerably smaller amount of waste develops.

The process according to the invention will be explained in more detailwith reference to the drawing on the basis of an embodiment by way ofexample.

FIG. 1 is a view showing a blade blank in cross-section produced, forexample, through a forging or cutting treatment;

FIG. 2 is a cross-sectional view showing the blank with a welded steelplate portion; and

FIG. 3 is a cross-sectional view showing the turbine blade obtainedafter drop forging, precisely to measurement, and subsequent heattreatment.

From a billet, pre-rolled in the customary manner, a correspondingsection is rough forged in such a manner that it can be shaped with asfew strokes as possible in a preliminary drop forge without too great aflow of material. This blank is freed, in the customary manner, of theflash under a trimming press while retaining the forging heat, and whichflash was formed from the excess material between the two halves of thedrop forge around the contour of the blank. Subsequently, thepreparation for a welded steel plate portion 2 takes place by cuttingout a groove 1 in parallelism to the longitudinal extent of the bladeblank on the leading edge area ('FIG. 1). This groove could be providedduring the drop forging; if not, it can be produced by material cuttingprocesses, for example, a cutting treatment or an electro-erosivetreatment.

One must be careful that the groove and the adjacent area of the blankare carefully descaled so that a good metallic interface, free from theinclusion of oxide, will occur with the hard weld alloy 2 that is to beapplied. Prior to the welding process, and depending on the materialcharacteristics of the basic raw material of the blank and portion to bewelded, which must be taken into con sideration, and also especially inconsideraton of sensitivity to hot tears, a careful preheating will takeplace, such as by resistance or induction heating. The application ofthe welding material to form the portion is accomplished by means ofknown fusion welding processes, whereby, if need be, welding under aprotective gas or under slag, with or without feeding of fluxing agentsfrom the casing of the electrode or through a welding powder can beutilized if the alloys require it.

After a careful cooling of the welded blanks, and in any case afterexposure to compensatory annealing for avoidance of tension cranks inthe base material, or particularly on the surface of the hard alloyportion, the uneven spots resulting from the welding beads in the weldportion can be smoothed, for example, by grinding. The blanks is theagain heated slowly and intensively, and is shaped to the finaldimensions in a finishing drop forge of precise measurements. The slightflash developing during this step is removed through some mechanicalprocess, for example, grinding, and the blade is then hardened byheat-treatment. After reheating to below the annealing temperature and,if need be, after interposing a leveling stroke in the final drop forge,a straightening step can take place for the elimination of anydistortion (warpage) which may possibly have occurred through thehardening process. For sensitive raw materials, it is recommended that aspecial Quette be used for aligning. Insignificant deviations from thesize tolerance, which still exists after removing (by polishing) thesuperficial accumulations of material, are eleminated by subsequentaligning in the cold state.

Compared to the processes customary heretofore, the

method of operation according to this invention has proven itself to besuperior for the simple reason that by way of the heat-treament aftersteel-facing by welding, a uniform structure, free of cracks, will beachieved. Also, there is no danger that the tension in the material willhave a disturbing etfect during operation, such as leading to subsequentchanges of shape during the heating of the finished blades to operatingtemperature. The inlet edge portions Welded on according to theinvention and shaped through subsequent forging, together with the basicblank, will have, in the case of the correct choice of the raw material,a superior surface, be free of cracks, corrosion resistant, andexceedingly resistant to the erosive stress resulting from the drivingmedium. Expansion joints or separation between the basic raw materialand the hard alloy has been welded is impossible, so that underwashescan no longer occur at the surface interface made their effectsdisturbingly felt in the case of edges produced according to othermethods, especially when soldering. It is particularly surprising that,contrary to all expectations corresponding to expert knowledge, forgingof these blanks provided with a welded steel plate por tion, preciselyaccording to measurement, is possible without difficulties even if, forexample, high-speed steels or hard alloys of the Co-Cr-W type are used,which in themselves are not considered forgeable. Another technicaladvantage which occurs, particularly with regard to the simplificationand the lowering of costs for the production process, lies in theavoidance of a cutting treatment of the edges made of hard alloy, whichis possible only through grinding and, even then, with difiiculties andonly by means of special equipment.

What is claimed is:

1. A method of producing turbine blades comprising the steps ofpreforrning a steel blade blank having a recess along the inlet edgethereof; fusion welding with a hard alloy steel to apply said hard alloysteel in excess to said recess; carefully cooling to prevent tensioncracks in said steel blade blank; mechanically smoothing any unevenspots resulting from welding beads; forging said steel blade blank andsaid welded hard alloy steel into a smooth turbine blade configurationof final shape; and then heating treating.

2. A method as claimed in claim 1 wherein said hard alloy steel weldmaterial is of the Co-Cr-W type; and said forging is drop forging.

3. A method as claimed in claim 1 comprising the further steps ofdescaling said steel blade blank and especially said recess, as well aspreheating said blank prior to said welding step.

References Cited UNITED STATES PATENTS 1,755,321 4/1930 Hendrickson416-224 3,215,511 11/1965 Chisholm et al. 416224X 3,088,192 5/1963Turner 29156.8 3,148,954 9/1964 Haas 159AUX 3,275,295 9/1966 Caldwell etal. 170--159X 3,304,056 2/1967 Akio Sohma 170159X FOREIGN PATENTS550,341 I/ 1943 Great Britain 170 -159A JOHN F. CAMPBELL, PrimaryExaminer D. C. REILEY, Assistant Examiner US. Cl. XJR.

